SM24A-01
The Role of the Plasmasphere in Solar-Wind/Magnetosphere Coupling
Under normal or low levels of geomagnetic activity, outgassing from the dayside ionosphere builds up in the middle magnetosphere to form the high-density, cool plasmasphere. When geomagnetic activity increases, this dense plasma is convected across the dayside magnetosphere where it flows into the dayside reconnection site. Because of its high mass density, this plasma lowers the Alfven velocity at the dayside reconnection site, slowing the flow of plasma and magnetic field lines through the site. This lowers the rate of reconnection between the magnetosphere and the solar wind, reducing the strength of solar-wind/magnetosphere coupling. Theoretical calculations, computer simulations, and statistical evidence of this reduced coupling is presented. Implications for a magnetospheric control of the solar-wind/magnetosphere coupling during special circumstances is discussed.
SM24A-02
Overview of Plasmaspheric Plumes With WHISPER/CLUSTER Observations
Plasmaspheric plumes have been routinely observed by CLUSTER. They are a common feature of the plasmasphere geometry and exhibit the high dynamics of this region. The CLUSTER mission provides high time resolution four-point measurements of the plasmasphere near perigee. Total electron density profiles are derived from the plasma frequency identified by the WHISPER instrument onboard CLUSTER. Plasmaspheric plumes are identified from density profiles determined along plasmasphere crossings as a significant density increase followed by a density decrease from the background value. Note that the WHISPER instrument has a limited frequency range (2-80 kHz); therefore only plasmaspheric plumes with densities below 80 cm-3 are listed in this study. Another limitation comes from the relatively high perigee of CLUSTER (4 RE), which means that the satellites cross the outer regions of the plasmasphere, which happen to be the most dynamic ones. We present a statistical analysis of plasmaspheric plumes observed by WHISPER onboard the four CLUSTER spacecraft during 5 years (1 February 2001 to 1 February 2006). Their occurrence is studied as a function of several geomagnetic indices (Kp, Dst and IMF); their transverse equatorial size, magnetic local time and magnetic latitude distribution is presented; comparisons of the density profiles of the same plume crossed in the Northern and Southern Hemisphere are also shown.
SM24A-03 INVITED
The role of the plasmasphere in the dynamics of ionospheric density structures
Coordinated ground-based and satellite observations are of great importance for understanding the electrodynamics of inner-magnetospheric density structures. Examples of these structures are storm enhanced density (SED) and plasmasphere drainage plumes resulting from the erosion of the plasmasphere boundary layer by sub-auroral disturbance electric fields. SED or localized enhancement of TEC results from a poleward redistribution of dusk-sector plasma from the low-latitude region during the early stage of a strong geomagnetic disturbance. While ground-based observations with GPS TEC and ISR combined with low-altitude satellite observations such as DMSP, TOPEX, and JASON see enhanced density structure, IMAGE EUV magnetospheric imagery from space detect this enhanced density as a pronounced bright region in the inner plasmapshere. In this paper, we present coordinated ground-based and satellite observations results showing the role of the plasmasphere and plasmapause in the dynamics of ionospheric density structures. This includes the simultaneous global observations of ground-based GPS TEC and ionospheric TEC observed by the JASON and TOPEX satellites to experimentally determine the percentage contribution of the plasmapshere to the ground- based GPS TEC and thus to the degradation of navigation and communication systems. We find the plasmasphere can contribute significantly to round-based GPS TEC.
SM24A-04
Comparison of EUV Brightness With Line-of-Sight Total Electron Content in the Earth's Plasmasphere.
Total electron content (TEC) data, obtained from radio signals sent between the Jason-1 satellite in low-Earth- orbit (LEO) and global positioning system (GPS) satellites, are inverted using tomography to infer electron density in Jason-1's orbital plane. We then construct column density traces along the lines of sight of an extreme ultraviolet (EUV) image of the He+ population of the plasmasphere, taken by NASA's IMAGE spacecraft. In this way, we produce column integrated electron density (TEC) values aligned with IMAGE's lines of sight. Since singly-ionized helium in the plasmasphere is optically thin, the radiance of an EUV image pixel is proportional to the column-integrated density of He+ ions along the line of sight. Hence, a TEC measurement gives us a column-integrated electron density, while an EUV image brightness value is proportional to a column-integrated He+ density. By comparing the radial brightness profile from an EUV image to the line-of-sight TEC in Jason's orbital plane, we can gain insight into the behavior of He+ and electron density in the plasmasphere. Future comparisons between TEC and EUV data could lead to an improved, three-dimensional, empirical model of plasmaspheric electron density, and a better understanding of IMAGE EUV images themselves.
SM24A-05 INVITED
The role of the plasmapause and plasma plumes in EMIC wave source location and propagation
It has generally been assumed in the past that the steep density gradient in the radial density profile of the magnetospheric plasma, the plasmapause, is the favoured region for the generation and propagation of electromagnetic ion cyclotron (EMIC) waves. This is the region of overlap between the expanding cold plasmasphere with the inner edge of the hot ring current during storm recovery, providing favourable conditions for EMIC instability. The plasmapause is also expected to provide a convenient gradient for guiding waves from equatorial sources to higher latitudes. Although EMIC waves are seen at the plasmapause, they have been seen more frequently by AMPTE and CRRES outside the plasmapause. More recently extended radial plasma plumes, attached to the plasmasphere have been seen in IMAGE-EUV data. These provide enhanced radial plasma density and associated azimuthal gradients. Using CRRES, GOES, LANL and IMAGE-EUV data the role of plasma gradients in determining EMIC source location and propagation properties will be considered using case studies and statistics. These results will provide an indication of the conditions under which EMIC waves generation occurs in the magnetosphere, and consequently their role in ring current ion loss.
SM24A-06 INVITED
The Role of the Plasmasphere in Controlling Relativistic Electron Precipitation
Resonant interaction with plasma waves is a primary cause of pitch-angle scattering and subsequent precipitation of radiation belt electrons. In the region outside the plasmasphere, whistler-mode chorus (~kHz) causes intense microburst precipitation, particularly during the main phase of geomagnetic storms. In the plume region near the plasmapause, EMIC waves (~Hz) may strongly scatter electrons with energies above ~1 MeV, leading to intense precipitation. Within the plasmasphere, plasmaspheric hiss (~100 Hz) is responsible for the creation of the slot region between the inner and outer radiation belts. This talk will briefly review the expected role of the plasmasphere in controlling the location of these waves and the resonance condition for relativistic electrons. We then present recent results investigating the location relative to the plasmasphere of observed duskside relativistic electron precipitation. Data from IMAGE EUV, POLAR EFI, and LANL MPA are used along with a simulation of the plasmasphere to determine how the evolution of the plasmasphere affects the spatial location and distribution of observed precipitation.
SM24A-07
The Role of the Plasmasphere in Radiation Belt Particle Energization and Loss
The plasmapause separates cold dense plasma in the inner magnetosphere from hot, low-density outer magnetosphere plasma. This boundary is very dynamic in response to changes in magnetospheric convection and other stormtime phenomena. The outer radiation belt is also dynamic during stormtime in terms of both radial location and energetic particle population. It is proposed that outer radiation belt particles are variously depleted and energized due to wave-particle interactions inside and outside the plasmasphere. Testing this hypothesis requires simultaneous observations of energetic particles and the plasmapause location. We use DMSP identifications of the plasmapause signature in the ionosphere (specifically the light ion trough) to derive plasmapause locations. This offers significantly improved temporal coverage given the continuous multiyear coverage by multiple DMSP satellites and the overlapping radiation belt observations by SAMPEX. The light ion trough location is semi-automatically identified from DMSP Retarding Potential Analyzer observations of light ion densities, then mapped along magnetic field lines to the plasmapause. Initial comparisons show good agreement between these plasmapause locations and those obtained from IMAGE EUV observations. Case studies also show good correlations between DMSP-identified plasmapause locations and SAMPEX observations of outer radiation belt particle distributions and precipitating particle microbursts. This approach will eventually provide an extensive database of plasmapause locations, permitting us to quantify the relationship between the light ion trough and the plasmapause, and improve understanding of the relationship between the plasmapause location and the outer radiation belt.
SM24A-08 INVITED
Energisation of Radiation Belt Electrons by ULF waves: Role of the Plasmapause and Heavy Ions
Recent theoretical studies have demonstrated how ULF waves can energise radiation belt electrons via inward transport through the drift-bounce resonance mechanism. The long-timescale behaviour is usually described as a diffusive inward transport in ULF wave fields. Given that ULF wave propagation is governed by the ambient mass density, the penetration of ULF wave fields to low-L in the radiation belts is influenced by the location of the plasmapause as well as heavy ion populations. We show examples from Halloween 2003 storms which demonstrate that changes to the background Alfven continnum, most likely resulting from the injection of heavy ions outside the plasmapause, can generate conditions preferential to the penetration of ULF power deep into the radation belts. Modelling of the drift resonant interaction demonstrates how time-limited ULF wavetrains can also lead to coherent electron transport on timescales much faster then expected through diffusion. During stormtimes, the ring current also penetrates to the heart of the outer radiation belt and can become dominated by 100's keV O+ ions outside the depleted plasmapause. We show that it is possible for moderately high azimuthal wavenumber (m) ULF waves to be driven by drift-bounce resonanace mechansism with these energetic ions. Under conditions of a depleted plasmasphere, these ring current ion excited ULF waves can resonate with radiation belt electrons. This provides an exciting potential pathway for the transfer of energy from the storm-time ring current into the radiation belts via the intermediary of ULF waves.